The present invention relates to a rear projection apparatus, and more particularly to a transmissive screen of high efficiency and low moire disturbance.
The configuration of a rear projection apparatus is shown in FIG. 1 (a plan view or a side view). In the figure, numeral 1 designates a screen, numeral 2 a projection lens, and numeral 3 a projection cathode-ray tube.
A prior-art example of the screen is shown in FIG. 2 (a perspective view). In the figure, numeral 2 designates the projection lens. Numerals 4 and 5 designate screen elements, which are respectively termed the "Fresnel sheet" and "front sheet". A concentrical Fresnel lens 20 is formed on the output side of the Fresnel sheet 4, and it transforms diverging input light into output light which is substantially normal to the plane of the screen. A horizontal lenticular lens 19 for horizontally diffusing light microscopically is formed on the input side of the front sheet 5. This front sheet is formed of a methacrylic resin, in which a diffusion material for diffusing light at random is mixed.
The performance of the screen of this prior art is featured by a gain G as viewed from the front side, the half gain angle .alpha..sub.H of horizontal directivity, and the half gain angle .alpha..sub.V of vertical directivity.
FIG. 3 is a perspective view for expressing the significances of .alpha..sub.H and .alpha..sub.V. In the figure, numeral 6 denotes the range of effective directivity.
The typical values of the screen attained by the prior art were G+5, .alpha..sub.H =40.degree. and .alpha..sub.V =6.degree.. Accordingly, the G.multidot..alpha..sub.H .multidot..alpha..sub.V product (the gain-half gain angle product) is calculated as 1200.
Meanwhile, the theoretical limit of a light transmission efficiency was analyzed as will be stated later. As a result, it has been found that the above value 1200 is an unsatisfactory level which is below 50% as compared with the theoretical limit. Besides, as the result of experimental study, it has been revealed that the deterioration is principally attributed to the diffusion material contained in the resin of the front sheet.
With the prior art, when the diffusion material is diminished, the vertical half gain angle .alpha..sub.V becomes too small, and moire disturbance arises. It has therefore been considered impracticable to attain enhancement in the gain by diminishing the diffusion material more.
The moire disturbance herein referred to is intended to mean an interference pattern as indicated at numeral 7 in FIG. 4 (a front view of the screen), which is generated by the interference between the concentrical pattern of the Fresnel lens and the vertical stripe pattern of the horizontal lenticular lens.
As regards a technique for reducing the moire disturbance of the projection screen, Japanese Patent Application Laid-open No. 191627/1982 entitled "Rear Projection Type Screen" states to the effect that setting the ratio between the pitch of the horizontal lenticular lens and the pitch of the Fresnel lens in a range of 1.35-1.43 is the best for the reduction of the moire disturbance. It has been difficult, however, to reduce the moire disturbance much more within or in the vicinity of this range. The principle of the appearance of the disturbance is, per se, unclear, and there has not been any example analyzed quantitatively.